1. Field of the Invention
The present invention relates to an electrostatic reproducing apparatus and, more particularly, to an electrostatic reproducing apparatus in which a plurality of color toners are employed to develop an electrostatic image on the surface of an image retainer. The invention also pertains to an electrostatic reproducing apparatus in which the image of a document is formed on the surface of a photosensitive member as an electrostatic latent image which is then developed by a developer and is transferred and fixed to transfer paper and, more particularly, to an electrostatic reproducing apparatus including an improved jamming sensor which detects any transfer paper which does not separate from the surface of the photosensitive member, the transfer paper having the developer image transferred thereto.
Further, the invention is concerned with an electrostatic reproducing apparatus which is arranged such that the supply of the developer or, other electrostatic reproducing process, for example, charging, exposing process is controlled in accordance with a judgement as to whether it is necessary to supply developer by detecting the density of the developer image (color toner image) formed on the surface of the photosensitive member.
2. Description of the Prior Art
Examples of conventionally known electrostatic reproducing apparatus of the type described above include a color picture image reproducing apparatus and a reproducing apparatus which is capable of reproduction with red or black toner.
As a means for detecting whether transfer paper having a toner image transferred thereto from the surface of an image retainer has been separated from the image retainer, a jamming sensing means has heretofore been known in which light is applied by a light-emitting element to the surface of the image retainer at a position where the separation of the transfer paper should have been completed, and the amount of light reflected thereby is detected by means of a light-receiving element, and, when the amount of the reflected light is relatively great, it is judged that the transfer paper has not been separated from the image retainer.
If such a jamming sensing means is applied to the electrostatic reproducing apparatus of the type described above, problems may arise. The amount of reflected light varies with the toner color; therefore, if a certain color toner is employed and the amount of reflected light is relatively great, it may be judged that the transfer paper has not been separated when in fact it has. Such misjudgement can be prevented by varying irradiation light and the light-receiving element with the toner color, but this makes the device more complex.
When a color toner is employed as a developer in an electrophotographic copying apparatus, a jamming sensor may deliver a detection output according to its spectral sensitivity when the amount of the light reflected from the developer remaining on the surface of a photosensitive drum is relatively great. This output is similar to that which is generated when the transfer paper has become undesirably wound on the surface of the photosensitive drum. In this case, transfer paper jamming cannot be accurately detected.
Such erroneous detection can be prevented by employing an arrangement in which the spectral sensitivity of the jamming sensor can be selected to match the toner. Then, however, the jamming sensor must be replaced with one which matches the toner.
This sensor detects the density of a reference picture image formed on the surface of the photosensitive drum. This reference picture image is formed by leading the picture image of a density reference plate to the photosensitive drum before the optical system reaches its predetermined exposure scanning speed and developing the image with a development unit. The developer density is controlled by an electromagnetic valve which supplies developer to the development unit hopper in response to a signal output from the density sensor which represents the density of the reference picture image.
This is known as the developer density control system by the picture image density detection method. The information detected by the density sensor includes variation in sensitivity of the photosensitive drum as the photosensitive member, change in the amount of charge in the developer, and the variations in charging and exposure conditions. It is, therefore, possible to control the developer density with respect to these variations and changes, thereby maintaining stable picture image density.
However, in a copying apparatus which is capable of employing a plurality of color toners as developer. Examples of multi-color copying apparatuses include one in which a plurality of development units, each filled with developer of a different color are arranged in the direction of rotation of the photosensitive drum, and one of the development units is selectively used. In another example, a plurality of such development units can replace each other. Since color toners have different spectral reflectance, a detector circuit which receives the output of the light-receiving element in the density sensor may be driven to saturation by certain wavelengths of irradiation light. This may also occur for a certain degree of spectral sensitivity of the light-receiving element in the density sensor. This may make it impossible to detect picture image density.
Techniques have recently been developed for forming color picture images in electrophotographic copying apparatuses with color toners which develop colors such as red, blue, green and sepia.
In an electrophotographic copying apparatus, development is generally effected such that toner particles are attached to an electrostatic latent image formed on the surface of an image retainer, such as a photosensitive drum, by exposing a document to light. The amount of toner attached to the latent image in this development greatly affects picture image quality. For this reason, to obtain a clear picture image which has adequate density and excellent color, it is necessary that the amount of the toner attached to the latent image in development be accurately controlled.
In the conventional picture image formed with black toner, an area exclusively for density detection is provided on a region of the surface of the image retainer, and what is called a "solid toner image" is formed in said detection area by an ordinary process, whereupon light for detetion is applied to the solid toner image from a light-emitting element, and the light reflected from the solid toner image is received by a light-receiving means including a light-receiving element. When the quantity of received light exceeds a predetermined level, a low-density signal is generated which indicates that the picture image density is low, and the toner is supplied to the development device in response to the low-density signal, thereby controlling the picture image density.
However, when an image is formed with color toners, if the density is controlled in the same manner as with black toner, a problem arises. Since colors have different spectral reflectance values for detected light, a color toner may cause a low-density signal to be generated despite the fact that the amount of the toner attached to the latent image is sufficient. This, may cause too much toner to be supplied, resulting in a fogged, low-quality picture image. Conversely, a color toner may not cause a low-density signal to be generated despite the fact that the amount of the toner attached to the latent image is insufficient, so that no toner supply may be effected, resulting in a picture image which has a low density and inferior color. Consequently, it is difficult to form a clear picture image with adequate density with color toners.